1. Field of Invention
The present invention is directed to trigger devices for actuating downhole tools and, in particular, trigger devices having a dissolvable material such that when the dissolvable material dissolves, the trigger device is activated and the downhole tool is actuated.
2. Description of Art
Some downhole tools need to be retained in an unset position until properly placed in the well. It is only when they are properly located within the well that the downhole tool is set. Such downhole tools in the past have had trigger mechanisms that are retained in an immovable position while the downhole tool is being “run” into the well and properly placed within the well. One prior technique for holding the trigger mechanism immobile until the downhole tool is properly placed in the well involves disabling the trigger with a mechanical device that is held against movement by a Kevlar® high strength fiber and an associated electrically powered heat source generally powered by stored batteries in the downhole tool. The generation of sufficient heat burns the fibers and releases the trigger so that the tool can set. Such a system is described in U.S. Pat. No. 5,558,153. One problem with this trigger mechanism is that it is extremely difficult to generate sufficient heat downhole to burn the fibers without damaging adjacent components. This is because the physical size of the battery pack must be large enough to provide sufficient energy to generate the necessary temperature, for the necessary duration, to break the fibers. Another issue is the very high temperatures needed to break the fibers and the effect on the overall design of the downhole tool from having to keep heat sensitive components away from the heated area.
Another prior trigger mechanism includes a battery operated heater coil in a downhole tool to release the trigger by applying heat and melting a plug to start the setting sequence. This design is reflected in U.S. Pat. No. 6,382,234. As with the other prior attempt, the size of the battery to provide the required electrical capacity to create enough heat to melt the plug presents a space concern in a downhole tool where space for a large power supply is at a premium. Further, the heat sensitive components must be shielded from the heater coil. The cost and the reliability of a large battery pack is also can be a problem. Additionally, safety is another issue because some batteries need special shipping and handling requirements.
Still other alternatives involve the large battery pack to accomplish a release of the trigger. For example, U.S. Pat. No. 5,558,153 also suggests using solder wire that melts at relatively low temperatures to be the trigger material or using the stored power in the battery to advance a knife to physically cut the fiber as opposed to breaking it with a battery operated heat source.
In other prior attempts, pressures from fluids pumped down the well are used to break shear pins on the downhole tools. The use of shear pins, however, requires elevated directional pressure forces acting on the shear pins. However, in some instances sufficient pressure may not be available. Alternatively, in some wells, pressure, even if available, cannot be utilized because additional intervention steps are required which results in the well experiencing undesirable “downtime” for the additional intervention steps. Additionally, in some instances, the shear pins fail to shear when they are supposed to, causing further delays.
Accordingly, prior to the present inventions trigger devices and methods for actuating downhole tools have been desired in the art which: permit customization of the trigger device such that the amount of time for the trigger device to be activated is pre-determined; permit setting of downhole tools without the need for high pressures or heat; allow the setting of the downhole tool without additional intervention steps and, thus, decreasing the costs associated with actuating the downhole tools.